cDNA libraries were prepared using mRNAs isolated from specific regions of the brain of the Rhesus monkey. Differential hybridizations were carried out to identify cDNA clones corresponding to genes predominantly expressed in either association or primary sensory neocortical areas. Total RNA was isolated from control and trisomy 16 mouse tissue. The polyadenylated fraction was isolated and used for Northern blot hybridization analysis to quantitate the expression of various genes located on mouse chromosome 16. The activity of the protein product (tyrosine protein kinase) of the src oncogene was measured in whole brain of Fischer-344 rats by an in vitro immune complex kinase assay. There was no significant difference in the pp60c-src specific kinase activity as a function of age. The gene for high-mobility-group (HMG) chromosomal Protein HMG-14, which is located in region 2lq22.3 of human chromosome 21 (obligated Down syndrome region) was identified on mouse chromosome 16; HMG-14 is expressed 1.5 times normal in mouse trisomy 16, a model for human trisomy 21. Increased numbers of extra-adrenal chromaffin cells of abdominal paraganglia of senescent Fischer-344 rats are not to due cell proliferation, but to induction from prerexisting cells via glucocorticoids.